Lead alloy products

ABSTRACT

A corrosion resistant lead alloy product having a thin surface layer of large elongated grains overlying a matrix of substantially smaller grains or crystals having random orientation.

United States Patent 1191 Mao [ Dec. 30, 1975 LEAD ALLOY PRODUCTS [75] Inventor: George W. Mao, St. Paul, Minn.

[73] Assignee: Gould Inc., Mendota Heights, Minn. [22] Filed: July 14, 1971 [21] Appl. No.: 162,605

Related US. Application Data [63] Continuation-impart of Ser. No. 749,257, July 25,

1968, abandoned.

52 us. c1. 148/3; 136/65; 148/13;

' 148/32; 148/39 51 1111.01 (1221 1/12; 1101111 35/08 58 Field ofsearch ..75/166,167;'148/2,3,

[56] References Cited UNITED STATES PATENTS 1,531,784 3/1925 Hazelett 148/32 1,780,261 11/1930 Corson 148/325 1,880,746 10/1932 Bouton 148/158 X 1,890,013 12/1932 Dean 148/158 OTHER PUBLICATIONS Research, Vol.8, No. 11, Nov. 1955, pp. 5 53 8!. S 54.

Auslegeschrift, 1084926, July 1960, 2 pages.

Primary ExaminerCharles N. Lovell Attorney, Agent, or Firm-Jacobson and Johnson [57] ABSTRACT A corrosion resistant lead alloy product having a thin surface layer of large elongated grains overlying a matrix of substantially smaller grains or crystals having random orientation.

8 Claims, 4 Drawing Figures US. Patent Dec. 30, 1975 Sheet 1 of2 3,929,513

Sheet 2 of 2 3,929,513

US. Patent Dec. 30, 1975 INVENTOR GEORGE 14 M40 LEAD ALLOY PRODUCTS RELATED INVENTIONS This application is a continuation-in-part of my application Ser. No. 749,257, filed July 25, 1968, now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to corrosion resistant lead alloy products and, more specifically, to corrosion resistant calcium-lead alloy products for use in pressure cast battery grids.

2. Description of the Prior Art Battery grids made from lead alloy products such as calcium-lead alloy-are well known in the art. These battery grids are generally gravity cast or pressure cast. An alloying element, such as calcium, provides the strength to support the battery grid while the lead provides the necessary electrochemical properties. The pressure cast method of producing battery grids is greatly preferred because pressure cast grids can be made faster and cheaper than gravity cast grids. However, pressure cast grids are susceptible to corrosion. Because of their susceptibility to corrosion, pressure cast battery grids have been used for negative grids while positive grids have been made from more corrosion-resistant gravity castings.

Generally, corrosion resistance is directly related to the area of the grain boundaries and the greater the grain boundary area, the more susceptible to corrosion is the grid. The narrow grain boundaries produced by fine grain structures, such as is normally found in pressure casting, should have superior corrosion resistance to the wider grain boundaries such as found in the large grain structures of gravity castings. However, the fine grain structure resulting from the rapid solidification of pressure casting produces highly-stressed, narrow grain boundaries that corrode much more rapidly than the stress-free wide boundaries found in gravity castings. Consequently, positive battery grids have been gravity cast to improve their corrosion resistance.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 in the accompanying drawings is a photomicrograph of a calcium-lead alloy produced in accordance with the invention and shows, in cross-section, a monograin layer along the surface of the heat-treated specimen;

FIG. 2 shows a portion of a battery grid;

FIG. 3 shows an enlarged portion of the grid surface shown in FIG. 2; and

FIG. 4 is a sectional view of the grid segment of FIG. 3.

SUMMARY I have discovered that by heating a pressure cast calcium-lead alloy battery grid at a prescribed elevated temperature for an extended period of time and then quenching the alloy in air at room temperature produces a battery grid having a monograin surface layer of high corrosion resistance yet maintains the desired structural hardness and grid strength.

DESCRIPTION OF THE PREFERRED EMBODIMENT Battery grids are well known in the art and although 5 they may take a variety of forms, shapes and sizes, they typically have a screen-like structure which is sufficiently strong to support the battery plates. Normally, some alloying element such as calcium or lithium are added to the lead to produce a lead alloy having sufficient structural strength to support both the grid and the active material applied to the grid. One such grid contains an amount of calcium ranging from 0.05 percent to 0.07 percent by weight in the lead.

These grids vary greatly in design, however, they generally have protrusions which can be connected to other grids or plates by welding or the like, or the grids can also be attached to the battery terminal post. For use in lead acid batteries, pure lead would provide the best electrochemical performance but experience has shown that grids of this nature are of insufficient strength for use in the ordinary lead-acid battery. Therefore, as previously mentioned, the lead must be alloyed with some type of hardening or strengthening agent such as calcium or lithium. The preferred method of manufacturing calcium-lead battery grids is by the pressure casting process. In this process, the lead alloy is forced into a mold under pressure thereby quickly producing a battery grid. Battery grids produced by pressure casting contain a fine grain structure in contrast to the large grain structure found in the slower cooled gravity castings. However, these pressure cast battery grids contain a large number of highly stressed grain boundaries where rapid corrosion occurs.

I have found that by heat-treating a calcium-lead alloy battery grid in a furnace at a temperature of approximately 500 F. for an extended period of time of approximately 6 hours and then quenching the battery grid in air at room temperature of approximately F. produces a battery grid having improved corrosion resistance.

The heat-treating and quenching of the battery grid greatly reduces the exterior grain boundary area by producing a surface layer of grains that are thinner, larger, and longer than the matrix of grains on the interior of the battery grid. Typically, this surface layer has a thickness ranging on the order from 0.6 to 1.2 mils. This type of surface layer is referred to herein as a monograin surface layer or a monocrystalline surface layer because the thickness of the layer is only one grain, i.e., only one layer of thin, elongated grains is formed along the outer surface.

Heat-treating a battery grid in accordance with my invention converts the small numerous surface grains into a significantly lesser number of thin larger surface grains thereby greatly reducing the grain boundary area. This phenomenon occurs only along the surface layer as the interior grains do not grow as rapidly in the elongated direction.

The reduction of the surface grain boundary area reduces significantly the susceptibility to corrosion which occurs at grain boundaries to thereby produce a battery grid having greatly enhanced corrosion resistance with little sacrifice of grid strength.

Referring to the drawings, FIG. 1 shows an actual photomicrograph of the grain structure of the exterior and the interior of the lead alloy product of my invention.

3 FIG. 2 shows a portion of a battery grid 10 employing my monograin surface layers. The battery grid shown is of the type having an offset web with rounded corners. Reference numeral 11 designates the various horizontal cross members of the grid which arestaggered or; offset to produce a stronger grid. A typical, radius, for the rounded corner may be 1/16 of an .inch for a grid having an opening of square inches. In order tomore fully understandlthe grid grain structure, a portion of grid 10designated by reference numeral 13 has been enlarged and shown in FIG. ,3 and.

FIG. 4. These figures more clearly show the relationship of the interior grain structure to theexterior grain structure, I

Referring to FIG. 3, the designated portion 13 of grid has been greatly enlarged toshow theappearance of the exteriorgrain structure on the surface of; the battery grid. The,-exterior grain structure comprises large grains having a characteristicdimension 1 andw. Di-

. mensions 1 and w arearbitrarily' assigned to indicate approximate dimensions in a mutually perpendicular axis. As the grain size is irregular and does not conform to a regular shape, it will be understood thepurpose of these dimensions is only for reference purposes.

F1 6, 3'. The "grid was sectioned on a plane parallel to thel pl the.drawing to show the thickness t of the 'riorgrains in a monograin surface layer. Note, theexterbr grains have 'a thickness designated by t which generallymany times smaller than the characset of grains which are considerably smaller than the" grains on the exterior of the battery grid. Obviously, these interior grains have not grown in the same proportion'as the exterior grains; however, because the larger grains with the correspondingly less' grain bound-,-

ary area areon the exterior of the grid, they shield the interior grains from corrosion.

In heat-treating my lead alloy product, it is necessary to heat-treat the product for a sufficient period of time to obtain 'the desired monograinsurface layer. Typically, a pressure cast product, having a cross-sectional thickness of about 0.175 inch to 0.180 inch requires heat treatment for a minimum of approximately 6 hours within the temperature range of about 450F to 550F. If the cross sectional thickness of the product is thicker the time'of heat treatment will increase. Likewise, if the cross sectional thickness of the product is thinner the time of heat treating will decrease. Also, for a specific product thickness and a temperature near the upper limit of the temperature range, the heat treating time will be less than what the temperatureis near the lower end of the temperature range.

The important consideration is that the time of heat treatment and the'temperature selected-for such heat treatment varies according to the cross-sectional thickness of the product. Thus, it is readily observed that the cross-sectional thickness will determine the time of heat treatment.

In addition, dueto the elevated temperature which is held for an extended period of time, stress is also re- F IG iis a sectional view of the battery grid shown ins 4 lieved along the remaining grain boundaries of the battery grid.

By heat treating at the elevated temperature of approximately 500 F. it has been found that the hardness lossis only on the order of 10 percent, while heating at temperatures of only 300 F. to 400 F. the hardness loss is approximately 30 percent to 40 percent.

Tests also revealed that the tensile strength of the battery grid'was' notappreciably affected'by heat-treating at the elevated temperature ranging from 450 F. to 550 F. for an extended period of time. 7

Experimental tests'were run at room temperature to determine the corrosion resistance of these heattreated and' non-heat-treated pressure cast calciumlead battery grids at various current densities and for different clacium-lead alloy-products. The tests were run at approximately 7'milliar'nps per square centimeter and 28 milliamps per square centimeter forfour weeks in a 1.1 l5specific gravity sulfuric acid electrolyte. The heat-treated lead alloy, having 0.07 percent calcium by weight, exhibited'the greatest corrosion resistance at the higher current density. Even at a current density of 7 milliamps per square centimeter the corrosion resistance of 500 F. heat-treatedlead alloy was considerably greater than the non-heat-treated lead alloy. Tests werealso conducted on heat-treated pressure cast and non-heat-treated gravitycast calcium lead alloy products. Aftercorrosion at 7'milliamps per square centimeter, microstructurephotographs of the large grain, gravity-cast' lead alloy samples and the heat-treated pressure cast lead alloy samples revealed the corrosion resistanceof the heat-treated pressure cast sample was superior to the corrosion resistanceof the large grain gravity-cast'sample because' of the presence of themonograin surface layer on the heat-treated pressure cast sample. v 1

I claim:

-1.. A methodj of producing an, essentiallyimon ograinedlayer along the outer surface of a pressure cast lead alloy product, comprising the stepsof:

a. alloying lead with calcium ranging from 0.05 per-,

cent to 0.07 percent by weight; a b. pressure casting the lead alloy to form a product{ c. annealing said product at a temperature in a range of 450 F.'to 550"F. for approximately sixhours, followed by d. quenching said-product. 2. The method of claim 1 in which the product is quenched in air at approximately'70 F.

3; The method of producing a corrosion resistant surface by relieving stressand increasing the'crystal size along the outer surface of-a pressure cast lead alloy product consisting of lead and calcium with calcium ranging from about 0.05 percent to about 0.07 percent by weight, which method comprises annealing the lead alloy product at a temperature in the range of 450 F. to 550. F. for sufficient time to induce growth of an outer surface layer on the order of0.6 to 1.2 mils thickness and having thin flat grains followed by quenching the lead alloy product in 'air at room temperature.

4. A method of claim 3 wherein said lead alloy product is annealed for a minimum of approximately 6 hours.

5. The method of claim.3 wherein said alloy product is quenched in air at a temperature ranging from 60 F. toF.

" 6. A mul'ti-grained pressure cast lead-calcium alloy product having at least two distinct sets of grains:

a first set of irregular shaped grains of predetermined size located on the interior of said product;

a second set of flat elongated grains located on the exterior of said product, said flat elongated grains substantially longer and wider than said irregular shaped grains; said flat, elongated grains coacting to thereby produce a surface layer on the exterior of said product having relatively few grain bound- 

1. A METHOD OF PRODUCING AN ESSENTIALLY MONOGRAINED LAYER ALONG THE OUTER SURFACE OF A PRESSURE CAST LEAD ALLOY PRODUCT, COMPRISING THE STEPS OF: A. ALLOYING LEAD WITH CALCIUM RANGING FROM 0.05 PERCENT TO 0.07 PERCENT BY WEIGHT; B. PRESSURE CASTING THE LEAD ALLOY TO FORM A PRODUCT; C. ANNEALING SAID PRODUCT AT A TEMPERATURE IN A RANGE OF 450* F. TO 550*F. FOR APPROXIMATELY SIX HOURS, FOLLOWED BY D. QUENCHING SAID PRODUCT.
 2. The method of claim 1 in which the product is quenched in air at approximately 70* F.
 3. The method of producing a corrosion resistant surface by relieving stress and increasing the crystal size along the outer surface of a pressure cast lead alloy product consisting of lead and calcium with calcium ranging from about 0.05 percent to about 0.07 percent by weight, which method comprises annealing the lead alloy product at a temperature in the range of 450* F. to 550* F. for sufficient time to induce growth of an outer surface layer on the order of 0.6 to 1.2 mils thickness and having thin flat grains followed by quenching the lead alloy product in air at room temperature.
 4. A method of claim 3 wherein said lead alloy product is annealed for a minimum of approximately 6 hours.
 5. The method of claim 3 wherein said alloy product is quenched in air at a temperature ranging from 60* F. to 80* F.
 6. A multi-grained pressure cast lead-calcium alloy product having at least two distinct sets of grains: a first set of irregular shaped grains of predetermined size located on the interior of said product; a second set of flat elongated grains located on the exterior of said product, said flat elongated grains substantially longer and wider than said irregular shaped grains; said flat, elongated grains coacting to thereby produce a surface layer on the exterior of said product having relatively few grain boundaries said second layer being on the order of 0.6 to 1.2 mils.
 7. The invention of claim 6 wherein said multi-grain product comprises lead alloyed with calcium in the range of about 0.05 percent to 0.07 percent by weight.
 8. The invention of claim 6 wherein said product comprises a battery grid. 